Method of surface-hardening steel workpieces in the form of bodies of revolution



United States Patent 0 M 3,167,460 METHOD OF SURFACE-HARDENING STEELWORKPIECES IN THE FORM OF BODIES 0F REVOLUTION Gerhard Seulen,Remscheid, Germany, assignor to Deutsche EdelstahlwerkeAktiengeseilschaft, Krefeld, Germany, and AllgemeineElcktricitats-Qesellschaft, Berlin-Grunewald, Germany No Drawing. FiledAug. 6, 1962, Ser. No. 214,846 Claims priority, application Germany,Aug. 18, 1961, D 36,838 4 Claims. (Cl. 148-143) The present inventionrelates to a method of surfacehardening steel workpieces in the form ofbodies of revolution.

A well known process is that of inductively heating workpieces made ofsteel, such as bolts, bearings for shafts, rolls, rollers and likeobjects, which have the form of bodies of revolution, and of thenquenching them for the purpose of hardening. The inductors employed forthis purpose are either of a kind comprising one or more turns whichcompletely embrace the workpiece or the section of the workpiece that isto be hardened, or they merely cover parts of the workpiece. When usingthe first kind of inductor rotation of the workpiece is not a necessity,but the latter kinds of inductor require that the workpiece should berotated about its axis to permit a strip of the desired width around theentire peripheral surface to be raised to hardening temperature.

Inductive heating has the particular operational advantage over othermethods of heating, such as by gas jets, by immersion in baths or infurnaces, that heating can be accomplished within the shortest possibletime by a high local concentration of power. As known, currents areinduced inside the workpiece at a depth which primarily depends upon thenature of the material and upon the frequency that is employed. Whenheating begins, this depth is shallow because the depth of penetrationis reduced by the high permeability of the material and by its lowelectrical resistivity at room temperature. The depth at which heat isgenerated does not increase until the Curie temperature has beenexceeded at which the permeability, as known, becomes unity. Despite theabove mentioned advantage of inductive heating, this phenomenon involvesa drawback from the point of view of subsequent hardening because it isa matter of considerable difiiculty to achieve the greater hardeningdepths which are needed for instance in the case of steel or cast coldrolls, working and backing rolls. Moreover, when hardening the rear axleshafts of motor vehicles the same trouble arises. In all these cases itis desirable that a sufi'icient depth, i.e. a sufficiently thick surfacelayer, should be raised to a temperature above the A0 point without atthe same time undesirably raising the temperature of the core. Since therequired depths of penetration could in the past be achieved only byprolonging the heating time it was unavoidable that the transfer of heatduring these longer times also raised the temperature of the core toundesirably high levels.

It is the object of the invention to avoid these drawbacks of inductiveheating when surface hardening steel workpieces which have the form ofbodies of revolution and which are raised to hardening temperature byheating the whole of their peripheral surface by rotating the workpieceabout its axis under an inductor which covers only part of the surfaceof the rotating workpiece which is then quenched. According to theinvention this can be achieved by controlling the depth of penetrationof the heat and hence the hardening depth by varying the relative speedbetween inductor and workpiece during rotation in the course of theheating process, said variation 3,167,460 Patented Jan. 26, 1965 beingin the direction of an acceleration. More partcularly, assuming a givenpower density of the inductor, the speed of rotation is at first keptsufiiciently low for the surface area located directly below theinductor to be raised at least to the critical transformationtemperature.

This temperature is then kept constant within close limits by increasingthe speed of rotation. The treatment proceeds roughly as follows:

The part of the rotating workpiece surface which emerges from underneaththe inductor can cool whilst turning through the remaining path ofrevolution and until it re-enters the range of the inductor byconductively transferring some of its heat to more inwardly locatedparts of the workpiece. When re-entering the range of the inductor thenext thermal pulse raises the temperature to the former level again orto a level slightly above it. This process repeats itself until thewhole of the surface has uniformly accepted the hardening temperature,when quenching may immediately follow. Shortly before quenching takesplace the speed of rotation should reach a maximum to produce adistribution of temperature of optimum uniformity.

In the course of progressive hardening an increase in the speed ofrotation is indicated when the period of time a portion of the workpieceis underneath the inductor tends to become too long from the point ofview of the overall temperature level already achieved, and thereforelikely to give rise to undesirably high temperature levels. It istherefore most desirable to raise the speed of rotation continuously tobetween 30 and 300 revolutions per minute before quenching takes place.

The effect of the proposed method is due to the fact that the heatsources inductively generated inside the workpiece are distributedthroughout a much greater depth at temperatures above the Curie point.Since a temperature above the Curie point is reached underneath theinductor when the speed of rotation is low, the production of a verypenetrative heating effect is ensured even when relatively highfrequencies are employed. In the method as hitherto practised thedistribution of the heat sources is confined to a layer which isextremely thin until the Curie point has been reached over the whole ofthe surface. The transfer of heat to regions located further inside isexclusively by conduction. On the other hand, the method'according tothe invention generates the heat sources in every surface section at adepth as if the workpiece had already been austenitic from thecommencement of heating.

The method proposed by the present invention has yet another advantageover hitherto known methods of hardening. As known, the short period ofheating in an inductive heating process often makes it extremelydifficult to bring the carbides into solution in such a way that asatisfactory martensite is formed after quenching. This applies moreparticularly to low or higher alloyed steels requiring a certain periodof time for transforma tion, which is not made available in inductiveheatng because of its rapidity. In the method according to the inventionthe carbides are dissolved each time the surface areas pass underneaththe inductor. These consecutive thermal pulses give rise to aparticularly fine grained martensite structure when the workpiece isfinally quenched.

For steels which are liable to develop hair cracks in low temperatureregions it is proposed in one embodiment of the invention to use a highspeed of rotation at the beginning of the heating process, first toraise the surface region which is subsequently to be hardened uniformlyaround the circumference to a temperature between 200 and 500 C. Thistemperature level is chosen according to the composition of the steelthat it to be hardened and particularly with reference to itssensitivity to rapid temr3 perature rises. The speed of rotation is thenreduced to raise each of the surface areas passing under the inductorfrom this temperature of 200 to 500 C. to a temperature level of say 850C. and then during its further rotation outside the inductor to cool thesame to a temperature above the initial temperature by transfer of heatto the interior. In order to prevent the final hardening temperaturefrom being exceed whilst the supply of energy remains constant the speedof rotation must be progressively accelerated also in this variant ofthe proposed method with a view to progressively shortening the time ofaction of the inductor on each surface section.

It has been found that the method of heating in this way ensures aconsiderable hardening depth, even when currents of high frequency areused, without any risk of the surface being overheated. The resultanthardened structure is a uniform fine grained martensite.

The method is performed with inductors which only partly cover theworkpiece. Preferably hairpin-shaped or rod-shaped inductors are usedextending axially along the rotating work or section of work. It is alsofeasible to provide several such inductors around the workpieceperiphery.

For controlling the speed of rotation of the workpiece a timingcontroller may be used which operates on the basis of empiricallydetermined values and which progressively increases the speed of themotor. Alternatively, control can be by reference to a temperaturefeeler located where the surface emerges from underneath the inductorand arranged to prevent a selected temperature, say 850 C., from beingexceeded at the point of emergence from the inductor by raising thespeed of rotation when the temperature rises excessively.

According to one embodiment of the invention a roll of a steel with 1%carbon and l.52% chromium and of and inches diameter is treated, using aloop-type inductor generating a power density of 0.3-1.5 kw. per squareinch. The roll is rotated at a speed of 2-4 revo lutions per minuteuntil the surface area located directly below the inductor has beenraised to the critical transformation temperature, i.e., 860 C. Thespeed of rotation of the roll is then continuously increased until ithas a speed of -40 revolutions per minute, and then it is immediatelyquenched by a dipping, spraying or other usual process.

According to another embodiment, where the object is of a steel liableto develop hair cracks, e.g., a steel with 04-05% carbon, 06-09%manganese, 02-04% silicon, 0.8-l.2% chromium and 0.5-1% nickel, in lowtemperature regions, the object is rotated first at a high speed, e.g.,at a speed of 200 revolutions per minute until the surface region whichis subsequently to be hardened is raised to a temperature between 200 C.and 500 C., e.g., 350 C. The speed of rotation is then reduced to 2-4revolutions per minute until the temperature level of 850 C. is reached.The object then rotates outside the influence .of the inductor to coolthe object to a temperature of said 450 C. by transfer of heat to theinterior of :the object. Then with the object again under the influenceof the inductor, its speed of rotation is progressivelyv accelerated to300 revolutions per minute to bring the object to the'hardeningtemperature whilst the supply of energy by the inductor remainsconstant, and the object is finally quenched.

What I claim is:

1. A method of surface-hardening a steel workpiece having the form of abody of revolution which can be hardened by heating to at least itscritical temperature and then quenching, comprising the steps of:subjecting said workpiece to the heating influence of induction heatingmeans having an inductive heating influence over only a portion of theworkpiece section to be hardened; eflecting relative rotationbetween-said workpiece and said induction heating means to therebysubject the entire Peripheryof the workpiece section to be hardened toinduction heating, said relative rotation being at a speed and for atime suflicient to heat substantially only the surface portion of saidworkpiece section .to be hardened to at least said criticaltransformation temperature; thereafter increasing the speed of relativerotation between said work piece and said induction heating means tothereby maintain the tem erature of said surface portion substantiallyconstant while increasing the depth of heating of said workpiece untilsaid heated depth is at least at said critical temperature; thenquenching said workpiece to thereby form the desired surface hardenedlayer.

2. A method of surface-hardening a steel workpiece having the form of abody of revolution which can be hardened by heating to at least itscritical temperature and then quenching, comprising the steps of:subjecting said workpiece to the heating influence of induction heatingmeans having an inductive heating influence over only a portion of theworkpiece section to be hardened; effecting relative rotation betweensaid workpiece and said induction heating means to thereby subject theentire periphery of the workpiece section to be hardened to inductionheating, said relative rotation being at a speed not exceeding thirtyrevolutions per minute and for a time sufficient to heat substantiallyonly the surface portion of said workpiece section to be hardened to atleast said critical transformation temperature; thereafter increasingthe speed of relative rotation between said workpiece and said inductionheating means to a speed which is at least seven times thefirst-mentioned speed of relative rotation to thereby maintain thetemperature of said surface portion substantially constant whileincreasing the depth of heating of said workpiece until said heateddepth is at least at said critical temperature; then quenching saidworkpiece to thereby form the desired surface hardened layer.

3. A method of surface-hardening a steel workpiece having the form of abody of revolution which can be hardened by heating to at least itscritical temperature and then quenching, comprising the steps of:subjecting said workpiece to the heating influence of induction heatingmeans having an inductive heating influence over only a portion of theworkpiece section to be hardened; effecting relative rotation betweensaidworkpiece and said induction heating means to thereby subject theentire periphcry of the workpiece section to be hardened to inductionheating, said relative rotation being at a speed such that the relativesurface speed between said workpiece and said induction heating meansdoes not substantially exceed the order of three-hundred inches perminute and for a time suiiicient to heat substantially only the surfaceportion of said workpiece section to be hardened to at least saidcritical transformation temperature; thereafter increasing the speed ofrelative rotation between said workpiece and said induction heatingmeans so that the relative surface speed between said workpiece and saidinduction heating is at least seven times the first-mentioned relativesurface speed to thereby maintain the temperature of said surfaceportion substantially constant while increasing the depth of heating ofsaid workpiece until said heated depth is at least at said criticaltemperature; then quenching said workpiece to thereby form the desiredsurface hardened layer.

4. A method of surface-hardening a steel workpiece having the form of abody of revolution which can be hardened by heating to at least itscritical temperature and then quenching, comprising the steps of:subjecting said workpiece to the heating influence of induction heatingmeans having an inductive heating influence over only a portion of theworkpiece section to be hardened; effectin relative rotation betweensaid workpiece and said induction heating means to thereby subject theentire periphery of the workpiece section to be hardened to induc tionheating, said relative-rotation being at a speed and for a timesufficient to uniformly heat the surface portion of said workpiecesection to be hardened to a temperature in the range of 200 C. to 500C.; thereafter continuing relative rotation between said workpiece andsaid induction heating means at a speed and for a time sufficient toheat substantially only the surface portion of said workpiece section tobe hardened to at least said critical transformation temperature;thereafter progressively increasing the speed of relative rotationbetween said workpiece and said induction heating means to betweenthirty and three-hundred revolutions per minute to thereby maintain thetemperature of said surface portion substantially constant whileincreasing the depth of heating of said work- References Cited by theExaminer UNITED STATES PATENTS 1,783,764 12/30 Adams 21910.4l 2,643,3256/53 Body et al 219-10.41 2,935,433 5/60 Pribyl 148l44 DAVID L. RECK,Primary Examiner.

1. A METHOD OF SURFACE-HARDENING A STEEL WORKPIECE HAVING THE FORM OF ABODY OF REVOLUTION WHICH CAN BE HARDENED BY HEATING TO AT LEAST ITSCRITICAL TEMPERATURE AND THE QUENCHING, COMPRISING THE STEPS OF:SUBJECTING SAID WORKPIECE TO THE HEATING INFLUENCE OVER ONLY A PORTIONOF THE WORKPIECE SECTION TO BE HARDENED; EFFECTING RELATIVE ROTATIONBETWEEN SAID WORKPIECE AND SAID INDUCTION HEATING MEANS TO THEREBYSUBJECT THE ENTIRE PERIPHERY OF THE WORKPIECE SECTION TO BE HARDENED TOINDUCTION HEATING, SAID RELATIVE ROTATION BEING AT A SPEED AND FOR ATIME SUFFICIENT TO HEAT SUBSTANTIALLY ONLY THE SURFACE PORTION OF SAIDWORKPIECE SECTION TO BE HARDENED TO AT LEAST SAID CRITICALTRANSFORMATION TEMPERATURE; THEREAFTER INCREASING THE SPEED OF RELATIVEROTATION BETWEEN SAID WORKPIECE AND SAID INDUCTION HEATING MEANS TOTHEREBY MAINTAIN THE TEMPERATURE OF SAID SURFACE PORTION SUBSTANTIALLYCONSTANT WHILE INCREASING THE DEPTH OF HEATING OF SAID WORKPIECE UNTILSAID HEATED DEPTH IS AT LEAST AT SAID CRITICAL TEMPERATURE; THENQUENCHING SAID WORKPIECE TO THEREBY FORM THE DESIRED SURFACE HARDENEDLAYER.